The present invention relates to an active overvoltage protection apparatus for a bidirectional power switch which has two back-to-back in series connected semiconductor switches in the xe2x80x9ccommon collector modexe2x80x9d topology.
A bidirectional power switch of the type described above is known from the publication xe2x80x9cNovel Solutions for Protection of Matrix Converter to Three Phase Induction Machinexe2x80x9d, printed in the Proceedings of the xe2x80x9cIEEE Industry Applications Society Annual Meetingxe2x80x9d, New Orleans, La., Oct. 5-9, 1997, pages 1447 to 1454. The circuit diagram of a such bidirectional switch in the xe2x80x9ccommon collector modexe2x80x9d topology is illustrated in more detail in FIG. 1. For comparison, FIG. 2 shows a bidirectional power switch in the xe2x80x9ccommon emitter modexe2x80x9d topology, which is likewise known from the aforementione mentioned publication. These two bidirectional power switches each have two semiconductor switches 4 and 6, which are connected back-to-back in series. In FIG. 1, these two semiconductor switches 4 and 6 are connected back-to-back in series in such a manner that the two collector connections are electrically conductively connected to one another. In FIG. 2, the circuit formed by the two semiconductor switches 4 and 6 are connected back-to-back in series in such a manner that their emitter connections are electrically conductively connected. Since the emitter connections are linked, this circuitry is referred to as the common emitter mode. Insulated gate bipolar transistors (IGBT) are used as the semiconductor switches 4 and 6, and each have a reverse diode. The internal topology can be seen at the accessible connections of the bidirectional power switch 2. In the bidirectional power switch with the xe2x80x9ccommon collector modexe2x80x9d topology shown in FIG. 1, the connections E1, E2, G1 and G2 are accessible on the power switch 2. In contrast to this, in the bidirectional power switch 2 with the xe2x80x9ccommon emitter modexe2x80x9d topology shown in FIG. 2, the connections C1, C2, G1 and G2 are accessible.
According to the aforementioned publication, bidirectional power switches are used in a matrix converter. A matrix converter is a self-commutating direct converter. This self-commutating direct converter is a voltage intermediate-circuit converter without an intermediate circuit. The bidirectional power switches are arranged in a 3xc3x973 switch matrix in the matrix converter. This arrangement of the bidirectional power switches results in three input phases being connected to three output phases. The actuation of the semiconductor switches or of the semiconductor switch in the power switches of the 3xc3x973 switch matrix in each case results in a current path being connected in a bidirectional manner, that is to say from the input to the output and vice versa. One phase of the matrix converter is an arrangement of three bidirectional power switches, which produces a connection from the three mains system phases to one output phase. An LC filter is also connected to the input connections of the matrix converter, and is linked on the input side to a three-phase mains system. This LC filter, which is also referred to as an input filter, isolates pulse-frequency oscillations from the mains system. The size of this filter depends on the pulse frequency of the matrix converter. This self-commutating direct converter offers the advantage that, by virtue of the topology, it can feed back into the mains system, and produces sinusoidal mains system currents by means of an appropriately designed control system.
In addition to the already mentioned embodiments of the bidirectional power switch, there is also a further embodiment, which can likewise be found in the aforementioned cited publication. This embodiment is a semiconductor switch which is integrated in a diode bridge.
Since the matrix converter has no passive freewheeling circuits such as a voltage intermediate-circuit converter, high reverse voltages occur across the semiconductor switches owing to the inductances which are present in the circuit, particularly in the case of pulse blocking generated on the basis of an EMERGENCY-OFF (switching off the actuation pulses of all the semiconductor switches). These overvoltages can also occur as a consequence of an incorrectly initiated commutation sequence or due to failure of the actuation of bidirectional power switches. The output circuit is always interrupted in these situations. The interruption in the output circuit in conjunction with the inductances which are present in the circuit causes the overvoltages, which may result in destruction of the semiconductor switches.
An overvoltage protection apparatus is known from the aforementioned publication xe2x80x9cNovel Solutions for Protection of Matrix Converter to Three Phase Induction Machinexe2x80x9d. This overvoltage protection apparatus has two 6-pulse diode bridges which are linked to one another on the DC-voltage side by means of a capacitor. On the AC-voltage side, the one 6-pulse diode bridge is connected to the input connections of the matrix converter. The other diode bridge is connected on the AC-voltage side to the output connections of the matrix converter. A resistor is connected electrically in parallel with the capacitor, and it discharges said capacitor. An LC filter is also connected to the input connections of the matrix converter, and is linked on the input side to a 3-phase mains system. This LC filter, which is also referred to as an input filter, isolates pulse-frequency oscillations from the mains system. The size of this filter depends on the pulse frequency of the matrix converter.
Any overvoltages which occur are rectified by the diode bridges, and are passed to the capacitor. This capacitor thus absorbs the commutation energy. This overvoltage protection apparatus, which is also the subject matter of U.S. Pat. No. 4,697,230, requires an initial charging circuit for the capacitor. This initial charging circuit is required to prevent overvoltages at twice the mains voltage from occurring when the matrix converter is switched on. Without initial charging, high peak currents likewise occur, which must be carried by the diodes in the diode bridge. The resistor is designed such that a predetermined amount of energy is discharged from the capacitor. This amount of energy depends on a predetermined difference between the mains system voltage and the capacitor voltage.
An overvoltage protection apparatus which has two 6-pulse diode bridges is also known from the publication xe2x80x9cPerformance of a two Steps Commutated Matrix Converter for AC-Variable-Speed Drivesxe2x80x9d, printed in the EPE ""99 Proceedings, Lausanne, September 1999, pages 1 to 9. Each of these two diode bridges has a capacitor on the DC-voltage side. These two capacitors are electrically connected in parallel. A zener diode and a pulse resistor are electrically connected in parallel with these two capacitors and are used to limit the voltage across the capacitors to a predetermined value. Furthermore, each bidirectional power switch has a varistor and a back-to-back series connected zener diode, by means of which the overvoltages across the bidirectional power switch are limited.
In the publication xe2x80x9cA Matrix Converter without Reactive Clamp Elements for an Induction Motor Drive Systemxe2x80x9d, by Axel Schuster, printed in IEEE, 1998, pages 714 to 720, a number of varistors are provided as the overvoltage protection apparatus. A varistor is connected electrically in parallel with each semiconductor switch with each bidirectional power switch in the 3xc3x973 switch matrix. These 18 varistors protect the 18 semiconductor switches of the nine bidirectional power switches against overvoltages.
When this overvoltage protection device is being used, the connection point of the two collector connections of the two back-to-back series connected semiconductor switches must be connected to the exterior in the bidirectional power switches in the common collector mode. It is also possible for the bidirectional power switch to be formed from individual semiconductor components. A varistor can be electrically connected in parallel with each semiconductor switch in a bidirectional power switch only if the collector connections, or their junction point, are or is accessible.
In the publication xe2x80x9cTheory and Design of a 30-hp Matrix Converterxe2x80x9d, printed in IEEE Transactions on Industry Applicationsxe2x80x9d, Volume 28, Issue 3, May/June 1992, pages 546 to 551, an RCD circuit for the transistor which is integrated in a diode bridge is used as the overvoltage protection apparatus for a bidirectional power switch. The energy which is fed into the capacitor in the RCD circuit is normally converted into heat in the resistor in the RCD circuit. This RCD circuit is also referred to as a snubber circuit. The stored energy can also be used to supply energy for actuation of the semiconductor switches. This over-voltage protection apparatus is less suitable for a bidirectional power switch as shown in FIGS. 1 or 2. Furthermore, this overvoltage protection apparatus requires physical volume, with the magnitude of this physical volume being dependent on the commutation energy.
A voltage clamping circuit is disclosed in the publication xe2x80x9cBeschaltung von SIPMOS-Transistorenxe2x80x9d [Circuit of SIPMOS Transistors], printed in xe2x80x9cSiemens Componentsxe2x80x9d, Volume 22, Issue 4, 1984, pages 157 to 159. This voltage clamping circuit is shown in more detail in FIG. 3 for a semiconductor switch 4, and is denoted by 8. This voltage clamping circuit 8 comprises a zener diode 10, in particular a high-voltage zener diode, which is also referred to as a transil diode, and a decoupling diode 12. This voltage clamping circuit 8 is connected between the collector connection C and the gate connection G of the semiconductor switch 4. The semiconductor switch 4 is an insulated gate bipolar transistor (IGBT) with a reverse diode. The decoupling diode 12 disconnects the voltage clamping circuit 8 from the gate connection G of the semiconductor switch 4 when the semiconductor switch 4 is switched on. When the semiconductor switch 4 is in the switched-off state, the semiconductor switch 4 is actuated automatically as soon as its collector-emitter voltage exceeds the sum of the zener voltage of the transil diode 10, the threshold voltage of the decoupling diode 12 and the gate-emitter threshold voltage. Any overvoltage which occurs across the semiconductor switch 4 is thus actively limited by this semiconductor switch 4, although losses occur in the semiconductor switch 1t and in the transil diode 10.
This active overvoltage protection apparatus can be used directly for a bidirectional power switch in the xe2x80x9ccommon emitter modexe2x80x9d topology (FIG. 2). This means that a voltage clamping circuit 8 is electrically connected in parallel with the collector-gate path in each of the two semiconductor switches 4 and 6 in the bidirectional power switch 2 in the common emitter mode. This can also be achieved without any major complexity since the required connections, the collector connection C and the gate connection G, are accessible.
This known voltage clamping circuit 8 cannot be used directly in a bidirectional power switch in the common collector mode. For circuit 8 to be used, the common collector connection must be connected to the exterior from the bidirectional power switch 2. This demands a special form of the bidirectional power switch, which may need to be produced on a customer-specific basis. Any deviation from standard components increases the price of a product on the market.
The present invention is based on the object of specifying an active overvoltage protection apparatus for a bidirectional power switch in the common collector mode. Since the bidirectional power switch in the common collector mode has an associated diode network which is linked to the accessible gate and emitter connections of the bidirectional power switch in such a manner that the function of a voltage clamping circuit is provided for each of its two semiconductor switches, any overvoltage which occurs is no longer absorbed by the overvoltage protection apparatus, but is actively limited by one of the two semiconductor switches in the bidirectional power switch. The polarity of the reverse voltage applied to the bidirectional power switch determines which of the two semiconductor switches in the bidirectional power switch will be automatically activated. The diode network as a circuit in a bidirectional power switch in the common collector mode results in an active overvoltage protection apparatus for such a power switch.
In another embodiment of the present invention, the overvoltage protection apparatus has a voltage measurement apparatus, two voltage comparison devices and two decoupling diodes. The voltage measurement apparatus is electrically connected to the emitter connections of the bidirectional power switch in the common collector mode, and its two outputs are each linked to an actual input of the two voltage comparison devices. On the output side, each voltage comparison device is linked by means of the decoupling diode and by means of a driver stage to a gate connection of the bidirectional power switch. A predetermined reference value is applied to the reference value input of each voltage comparison device. If the actual value which is determined is greater than or equal to the predetermined reference value, the associated semiconductor switch in the bidirectional power switch in the common collector mode is actuated, so that this semiconductor switch can actively limit the overvoltage which occurs. In a preferred embodiment , a driver stage is connected between each of the junction points of the series circuits and the gate connections of the bidirectional power switch in the common collector mode. This greatly reduces the current load on each of the zener diodes.
One embodiment of the diode network, has two zener diodes and two decoupling diodes. one zener diode and one decoupling diode are in each case electrically conductively connected to one another on the anode side. Each of these series circuits is linked to a gate connection and to an emitter connection of in each case one semiconductor switch in the bidirectional power switch. Each series circuit thus forms a known voltage clamping circuit, although this is not electrically connected to the collector-gate path of a semiconductor switch. These two voltage clamping circuits are connected, crossed over, to the accessible connections of the bidirectional power switch-n the common collector mode.
In another embodiment of the diode network, the network likewise has two zener diodes and two decoupling diodes, with one zener diode and one decoupling diode each being linked to one another on the cathode side. These junction points are each connected to a gate connection of the bidirectional power switch. These two series circuits are electrically linked to one another in such a manner that the two zener diodes are electrically conductively connected on the anode side. The associated decoupling diodes are each connected on the anode side to an emitter connection of the bidirectional power switch. In contrast to the first embodiment, the decoupling diodes need not be high blocking capability diodes.
In a further embodiment of the diode network, the network has two decoupling diodes and one zener diode, which is integrated in a diode bridge. The free connections of this diode bridge are electrically conductively connected to the gate connections of the bidirectional power switch in the common collector mode. The decoupling diodes are each connected electrically in parallel, but in the reverse direction, to a gate-emitter path of a semiconductor switch in the bidirectional power switch in the common collector mode. This refinement of the diode network now requires only one zener diode, in particular a high blocking capability zener diode, thus significantly reducing the complexity and the costs of the active overvoltage protection apparatus.
In yet another embodiment of the diode network, the network has four diodes and one zener diode. On the anode side, this zener diode is linked to the anodes of the two diodes which, for their part, are connected to the gate connections of the bidirectional power switch. On the cathode side, the zener diode is connected to the cathodes of two diodes which, for their part, are linked to the emitter connections of the bidirectional power switch. In this embodiment of the diode network not only is one high blocking capability zener diode saved, but also two decoupling diodes. The complexity for an active overvoltage protection apparatus for a bidirectional power switch in the common collector mode has thus been further simplified.